Post Frontal Orographic Snowfall Impacts NM

A slow-moving upper level storm system tracked east across northern NM and southern CO on 14-15 December 2015. A weak tap of subtropical moisture ahead of this system provided light to moderate snowfall mainly along the Continental Divide of western NM and the higher terrain running north-south through central and northern NM. Snow accumulations of 3 to 8 inches were reported ahead of and immediately behind the surface front and the mid level trough passage. A classic westerly,upslope flow event developed behind the upper wave as moist, unstable flow interacted with the north-south oriented higher terrain. Winter weather advisories and winter storm warnings were in effect over much of northern NM for the expectation of storm total snowfall of 8 to 12″ with locally higher amounts. Figure 1 depicts the distribution of advisories and warnings over northern NM on the Albuquerque National Weather Service public page.

Advisory and warning map for the ABQ CWA valid 15 December 2015.

Figure 1. Advisory and warning map for the ABQ County Warning Area valid 15 December 2015.

Poor radar coverage over northern and western NM makes it a challenge for assessing winter precipitation patterns and snowfall rates. Figure 2 shows a radar mosaic valid 1800 UTC 15 December 2015 utilizing an enhanced color curve to identify areas of lighter snowfall. Automated surface observations are sparse in this area however there are at least a few observations reporting snow where nothing is present in the radar reflectivity. Webcams at ski resorts serve as an excellent near real-time proxy for visualizing active snow accumulations in these poor radar coverage regions. Additionally, once daily snow accumulation reports from ski resorts aid the verification process following the winter event.

Figure 2. Winter radar mosaic from KABX valid 1800 UTC 15 December 2015. Note the orange circle depicting a large area of poor radar coverage.

The integration of satellite data allows forecasters to supplement these data void areas. The most recent interation of the NESDIS snowfall rate products available at WFO Albuquerque illustrate the snowfall rate derived from radar (Figure 3a) and the snowfall rate available from merging the POES satellite data with the radar data (Figure 3b). Note the grey areas overlaid on the map in Figure 3a indicate areas of reliable radar coverage. The snowfall rate derived from satellite data in Figure 3b clearly shows coverage outside of the area with reliable radar coverage. A very cold and unstable airmass in association with this precipitation suggested snowfall rates in the higher terrain would average between 20-30:1. The 18:1 image in the lower right of Figure 3b indicated rates around 0.4/hr.

FIgure 3a. Radar derived snowfall rate product over northern NM valid 1750 UTC 15 December 2015.

Figure 3a. Radar derived snowfall rate product over northern NM valid 1750 UTC 15 December 2015. Note the grey areas overlaid on the map indicating where reliable radar coverage exists. Upper left (liquid equivalent), upper right (10:1), lower right (18:1), lower left (36:1).

NESDIS snowfall rate product filling in the radar gaps over northern NM valid 1750 UTC 15 December 2015. Note the circles in the upper left image are the location of the webcams in Figure 4.

Although there is sparse coverage of automated surface observations around the higher terrain, webcams from ski resorts can verify the existence of moderate to heavy snowfall. Visibilities in the webcams below suggest snowfall rates higher than those depicted in the NESDIS products – visually, rates look closer to perhaps 1″/hr in the upper right and lower right images (Figure 4). One of our goals of this assessment is to combine information from the webcams with the more quantitative snowfall rate product to better estimate snowfall in data void areas. Snowfall reports from the Chama Railyard indicated 8.5″, Taos Ski Valley 6″, Ski Santa Fe 12″, and Pajarito Mountain 10″.

 

Figure 4. Webcams from across northern NM. Top left (Chama Railyard, yellow circle), Top right (Taos Ski Valley, white circle), Bottom right (Ski Santa Fe, red circle), Bottom left (Pajarito Mt, orange circle).

 

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